共查询到20条相似文献，搜索用时 953 毫秒
1.
We present a kinematic, self-adaptive, numerical model to describe the down-flow thermal and rheological evolution of channel-contained lava. As our control volume of lava advances down a channel it cools and crystallizes, an increasingly thick and extensive surface crust grows, and its heat budget and rheology evolve. By estimating down-flow heat and velocity loss, our model calculates the point at which the control volume becomes stationary, giving the maximum distance lava flowing in the channel can extend. Modeled effusion rates, velocities, widths, surface crust parameters, heat budget, cooling, temperature, crystallinity, viscosity, and yield strength all compare well with field data collected during eruptions at Mauna Loa, KOlauea, and Etna. Modeled lengths of 25-27, 2.5-5.7, and 0.59-0.83 km compare with measured lengths of 25-27, 4, and 0.75 km for the three flows, respectively. Over proximal flow portions we calculate cooling, crystallization, viscosity, and yield strength of 1-10°C km-1, 0.001-0.01 volume fraction km-1, 103-104 Pa s, and 10-3-102 Pa, respectively. At the flow front, cooling, crystallization, viscosity, and yield strength increase to >100°C km-1, 0.1 volume fraction km-1, 106-107 Pa s, and 103-104 Pa, respectively, all of which combine to cause the lava to stop flowing. Our model presents a means of (a) analyzing lava flow thermo-rheological relationships; (b) identifying important factors in determining how far a channel-fed flow can extend; (c) assessing lava flow hazard; and (d) reconstructing flow regimes at prehistoric, unobserved, or remote flows. 相似文献
2.
3.
The initial cooling of pahoehoe flow lobes 总被引：1，自引：0，他引：1
In this paper we describe a new thermal model for the initial cooling of pahoehoe lava flows. The accurate modeling of this
initial cooling is important for understanding the formation of the distinctive surface textures on pahoehoe lava flows as
well as being the first step in modeling such key pahoehoe emplacement processes as lava flow inflation and lava tube formation.
This model is constructed from the physical phenomena observed to control the initial cooling of pahoehoe flows and is not
an empirical fit to field data. We find that the only significant processes are (a) heat loss by thermal radiation, (b) heat
loss by atmospheric convection, (c) heat transport within the flow by conduction with temperature and porosity-dependent thermal
properties, and (d) the release of latent heat during crystallization. The numerical model is better able to reproduce field
measurements made in Hawai'i between 1989 and 1993 than other published thermal models. By adjusting one parameter at a time,
the effect of each of the input parameters on the cooling rate was determined. We show that: (a) the surfaces of porous flows
cool more quickly than the surfaces of dense flows, (b) the surface cooling is very sensitive to the efficiency of atmospheric
convective cooling, and (c) changes in the glass forming tendency of the lava may have observable petrographic and thermal
signatures. These model results provide a quantitative explanation for the recently observed relationship between the surface
cooling rate of pahoehoe lobes and the porosity of those lobes (Jones 1992, 1993). The predicted sensitivity of cooling to
atmospheric convection suggests a simple field experiment for verification, and the model provides a tool to begin studies
of the dynamic crystallization of real lavas. Future versions of the model can also be made applicable to extraterrestrial,
submarine, silicic, and pyroclastic flows.
Received: 26 November 1994 / Accepted: 1 December 1995 相似文献
4.
A dynamical model of lava flows cooling by radiation 总被引：1，自引：0，他引：1
Michele Dragoni 《Bulletin of Volcanology》1989,51(2):88-95
The behaviour of a lava flow is reproduced by a two-dimensional model of a Bingham liquid flowing down a uniform slope. Such a liquid is described by two rheological parameters, yield stress and viscosity, both of which are strongly temperature-dependent. Assuming a flow rate and an initial temperature of the liquid at the eruption vent, the temperature decrease due to heat radiation and the consequent change in the rheological parameters are computed along the flow. Both full thermal mixing and thermal unmixing are considered. The equations of motion are solved analytically in the approximation of a slow downslope change of the flow parameters. Flow height and velocity are obtained as functions of the distance from the eruption vent; the time required for a liquid element to reach a certain distance from the vent is also computed. The gross features of observed lava flows are reproduced by the model which allows us to estimate the sensitivity of flow dynamics to changes in the initial conditions, ground slope and rheological parameters. A pronounced increase in the rate of height increase and velocity decrease is found when the flow enters the Bingham regime. The results confirm the observation according to which lava flows show an initial rapid advance, followed by a marked deceleration, while the final length of a flow is such that the Graetz number is in the order of a few hundreds. 相似文献
5.
Measurements of the anisotropy of magnetic susceptibility (AMS) of natural lavas have shown that AMS varies with depth within a lava flow. We have investigated the reasons for such variation by studying the effects of temperature and strain rate on the AMS of recent lava in the laboratory. Samples of lava from Kilauea were melted and subjected to a range of strain rate and cooling histories. The results show that the degree of anisotropy is a function of both the thermal and shearing history of a sample. High degrees of anisotropy were found only in samples that were deformed at temperatures close to those encountered during eruption and then rapidly quenched. Lavas subjected to similar shear stresses at high temperatures had low degrees of anisotropy if allowed to cool down slowly without further deformation. Additionally, lava subjected to complex shearing yield a lower degree of anisotropy even when high strain rates were imposed on it. These results lead to the conclusion that only the last phase of deformation is detectable using AMS and that high strain rates will not result in high degrees of anisotropy if either deformation ends while lava is still fluid or if the orientation of the maximum shear stress varies with time. The relation between the orientation of the principal susceptibilities and that of shear is less sensitive to variation on shear with time. Consequently, flow directions can be inferred confidently with this type of measurements. 相似文献
6.
Ciro Del Negro Luigi Fortuna Alexis Herault Annamaria Vicari 《Bulletin of Volcanology》2008,70(7):805-812
Since the mechanical properties of lava change over time, lava flows represent a challenge for physically based modeling.
This change is ruled by a temperature field which needs to be modeled. MAGFLOW Cellular Automata (CA) model was developed
for physically based simulations of lava flows in near real-time. We introduced an algorithm based on the Monte Carlo approach
to solve the anisotropic problem. As transition rule of CA, a steady-state solution of Navier-Stokes equations was adopted
in the case of isothermal laminar pressure-driven Bingham fluid. For the cooling mechanism, we consider only the radiative
heat loss from the surface of the flow and the change of the temperature due to mixture of lavas between cells with different
temperatures. The model was applied to reproduce a real lava flow that occurred during the 2004–2005 Etna eruption. The simulations
were computed using three different empirical relationships between viscosity and temperature. 相似文献
7.
Causes and consequences of pressurisation in lava dome eruptions 总被引：3，自引：0，他引：3
High total and fluid pressures develop in the interior of high-viscosity lava domes and in the uppermost parts of the feeding conduit system as a consequence of degassing. Two effects are recognised and are modelled quantitatively. First, large increases in magma viscosity result from degassing during magma ascent. Strong vertical gradients in viscosity result and large excess pressures and pressure gradients develop at the top of the conduit and in the dome. Calculations of conduit flow show that almost all the excess pressure drop from the chamber in an andesitic dome eruption occurs during the last several hundred metres of ascent. Second, microlites grow in the melt phase as a consequence of undercooling caused by gas loss. Rapid microlite growth can cause large excess fluid pressures to develop at shallow levels. Theoretically closed-system microlite crystallization can increase local pressure by a few tens of MPa, although build up of pressure will be countered by gas loss through permeable flow and expansion by viscous flow. Microlite crystallization is most effective in causing excess gas pressures at depths of a few hundred metres in the uppermost parts of the conduit and dome interior. Some of the major phenomena of lava dome eruptions can be attributed to these pressurisation effects, including spurts of growth, cycles of dome growth and subsidence, sudden onset of violent explosive activity and disintegration of lava during formation of pyroclastic flows. The characteristic shallow-level, long-period and hybrid seismicity, characteristic of dome eruptions, is attributed to the excess fluid pressures, which are maintained close to the fracture strength of the dome and wallrock, resulting in fluid movement during formation of tensile and shear fractures within the dome and upper conduit. 相似文献
8.
Cooling and crystallization of lava in open channels, and the transition of Pāhoehoe Lava to 'A'ā 总被引：1，自引：1，他引：0
Samples collected from a lava channel active at Kīlauea Volcano during May 1997 are used to constrain rates of lava cooling
and crystallization during early stages of flow. Lava erupted at near-liquidus temperatures (∼1150 °C) cooled and crystallized
rapidly in upper parts of the channel. Glass geothermometry indicates cooling by 12–14 °C over the first 2 km of transport.
At flow velocities of 1–2 m/s, this translates to cooling rates of 22–50 °C/h. Cooling rates this high can be explained by
radiative cooling of a well-stirred flow, consistent with observations of non-steady flow in proximal regions of the channel.
Crystallization of plagioclase and pyroxene microlites occurred in response to cooling, with crystallization rates of 20–50%
per hour. Crystallization proceeded primarily by nucleation of new crystals, and nucleation rates of ∼104/cm3s are similar to those measured in the 1984 open channel flow from Mauna Loa Volcano. There is no evidence for the large nucleation
delays commonly assumed for plagioclase crystallization in basaltic melts, possibly a reflection of enhanced nucleation due
to stirring of the flow. The transition of the flow surface morphology from pāhoehoe to 'a'ā occurred at a distance of 1.9 km
from the vent. At this point, the flow was thermally stratified, with an interior temperature of ∼1137 °C and crystallinity
of ∼15%, and a flow surface temperature of ∼1100 °C and crystallinity of ∼45%. 'A'ā formation initiated along channel margins,
where crust was continuously disrupted, and involved tearing and clotting of the flow surface. Both observations suggest that
the transition involved crossing of a rheological threshold. We suggest this threshold to be the development of a lava yield
strength sufficient to prevent viscous flow of lava at the channel margin. We use this concept to propose that 'a'ā formation
in open channels requires both sufficiently high strain rates for continued disruption of surface crusts and sufficient groundmass
crystallinity to generate a yield strength equivalent to the imposed stress. In Hawai'i, where lava is typically microlite
poor on eruption, these combined requirements help to explain two common observations on 'a'ā formation: (a) 'a'ā flow fields
are generated when effusion rates are high (thus promoting crustal disruption); and (b) under most eruption conditions, lava
issues from the vent as pāhoehoe and changes to 'a'ā only after flowing some distance, thus permitting sufficient crystallization.
Received: 3 September 1998 / Accepted: 12 April 1999 相似文献
9.
Natrocarbonatitic magmas are characterized by their extremely low viscosities and fast elemental diffusion, and as a consequence
of this, their chemistry and crystallinity can change significantly during residence in shallow reservoirs or even due to
cooling during lava flow emplacement. Here, we present the results of a series of crystallization experiments conducted at
1-atm confining pressure and in a temperature range between 630°C and 300°C. The experiments were set up to characterize the
chemistry and growth processes of the phenocryst phases present in natrocarbonatites. The results are applicable to (1) processes
occurring during residence in shallow magma reservoirs and/or (2) during lava flow emplacement. We show that during crystallization
of natrocarbonatites at atmospheric pressure, gregoryite is the first mineral to crystallize at 630°C, followed by nyerereite
at 595°C. Crystal size distributions of the gregoryites show that the crystals grow rapidly by textural coarsening (i.e.,
Ostwald ripening). As the crystallization is a continuous process at this pressure, the composition of the residual melt changes
in response to the crystallization. However, the experiments also show that individual crystals completely reequilibrate with
the changes in melt composition in as little time as <11 min. We therefore conclude that crystallization and diffusion are
extremely fast processes in the natrocarbonatitic system and that the measured chemical variations in phenocrysts from Oldoinyo
Lengai can be explained by different cooling histories. Finally, we model the rheological control on the emplacement of highly
crystallized natrocarbonatitic lavas at Oldoinyo Lengai. 相似文献
10.
Roberto Scandone Lisetta Giacomelli Francesca Fattori Speranza 《Journal of Volcanology and Geothermal Research》2008,170(3-4):167-180
During the period 1631–1944, Vesuvius was in persistent activity with alternating mild strombolian explosions, quiet effusive eruptions, and violent strombolian eruptions. The major difference between the predominant style of activity and the violent strombolian stages is the effusion rate. The lava effusion rate during major eruptions was in the range 20–100 m3/s, higher than during mild activity and quiet effusion (0.1–1 m3/s). The products erupted during the mild activity and major paroxysms have different degree of crystallization. Highly porphyritic lava flows are slowly erupted during years-long period of mild activity. This activity is fed by a magma accumulating at shallow depth within the volcanic edifice. Conversely, during the major paroxysms, a fast lava flow precedes the eruption of a volatile-rich, crystal-poor magma. We show that the more energetic eruptions are fed by episodic, multiple arrival of discrete batches of magma rising faster and not degassing during the ascent. The rapidly ascending magma pushes up the liquid residing in the shallow reservoir and eventually reaches the surface with its full complement of volatiles, producing kilometer-high lava fountains. Rapid drainage of the shallow reservoir occasionally caused small caldera collapses. The major eruptions act to unplug the upper part of the feeding system, erupting the cooling and crystallizing magma. This pattern of activity lasted for 313 y, but with a progressive decrease in the number of more energetic eruptions. As a consequence, a cooling plug blocked the volcano until it eventually prevented the eruption of new magma. The yearly probability of having at least one violent strombolian eruption has decreased from 0.12 to 0.10 from 1944 to 2007, but episodic seismic crises since 1979 may be indicative of new episodic intrusions of magma batches. 相似文献
11.
Andrea Borgia Scott Linneman Daniel Spencer Luis Diego Morales Jose Brenes Andre 《Journal of Volcanology and Geothermal Research》1983,19(3-4)
Arenal Volcano has effused basaltic andesite lava flows nearly continuously since September, 1968. The two different kinds of material in flows, lava and lava debris, have different rheologic properties and dynamic behavior. Flow morphology depends on the relationship between the amount and distribution of the lava and the debris, and to a lesser extent the ground morphology.Two main units characterize the flows: the channel zone and the frontal zone. The channel zone consists of two different units, the levées and the channel proper. A velocity profile in the channel shows a maximum value at the plug where the rate of shear is zero, and a velocity gradient increasing outward until, at the levées, the velocity becomes zero. Cooling produces a marked temperature gradient in the flow, leading to the formation of debris by brittle fracture when a critical value of shear rate to viscosity is reached. When the lava supply ceases, much of this debris and part of the lava is left behind after the flow nucleus drains out, forming a collapsed channel.Processes at the frontal zone include levée formation, debris formation, the change in shape of the front, and the choice of the flow path. These processes are controlled primarily by the rheological properties of the lava.Frontal zone dynamics can be understood by fixing the flow front as the point of reference. The lava flows through the channel into the front where it flows out into the levées, thereby increasing the length of the channel and permitting the front to advance. The front shows a relationship of critical height to the yield strength (τ0) surface tension, and slope; its continued movement is activated by the pressure of the advancing lava in the channel behind. For an ideal flow (isothermal, homogeneous, and isotropic) the ratio of the section of channel proper to the section of levées is calculated and the distance the front will have moved at any time tx can be determined once the amount of lava available to the front is known. Assuming that the velocity function of the front {G(t)} during the collapsing stage is proportional to the entrance pressure of the lava at the channel-front boundary, an exponential decrease of velocity through time is predicted, which shows good agreement with actual frontal velocity measurements taken on two flows. Local variations in slope have a secondary effect on frontal velocities.Under conditions of constant volume the frontal zone can be considered as a machine that consumes energy brought in by the lava to perform work (front advancement). While the front will use its potential energy to run the process, the velocity at which it occurs is controlled by the activation energy that enters the system as the kinetic energy of the lava flowing into the front. A relation for the energy contribution due to frontal acceleration is also derived. Finally the entrance pressure, that permits the front to deform, is calculated. Its small value confirms that the lava behaves very much like a Bingham plastic. 相似文献
12.
Using constraints from an extensive database of geological and geochemical observations along with results from fluid mechanical studies of convection in magma chambers, we identify the main physical processes at work during the solidification of the 1959 Kilauea Iki lava lakes. In turn, we investigate their quantitative influence on the crystallization and chemical differentiation of the magma, and on the development of the internal structure of the lava lake. In contrast to previous studies, vigorous stirring in the magma, driven predominately by the descent of dense crystal-laden thermal plumes from the roof solidification front and the ascent of buoyant compositional plumes due to the in situ growth of olivine crystals at the floor, is predicted to have been an inevitable consequence of very strong cooling at the roof and floor. The flow is expected to have caused extensive but imperfect mixing over most of the cooling history of the magma, producing minor compositional stratification at the roof and thermal stratification at the floor. The efficient stirring of the large roof cooling is expected to have resulted in significant internal nucleation of olivine crystals, which ultimately settled to the floor. Additional forcing due to either crystal sedimentation or the ascent of gas bubbles is not expected to have increased significantly the amount of mixing. In addition to convection in the magma, circulation driven by the convection of buoyant interstitial melt in highly permeable crystal-melt mushes forming the roof and the floor of the lava lake is envisaged to have produced a net upward flow of evolved magma from the floor during solidification. In the floor zone, mush convection may have caused the formation of axisymmetric chimneys through which evolved magma drained from deep within the floor into the overlying magma and potentially the roof. We hypothesize that the highly evolved, pipe-like ‘vertical olivine-rich bodies’ (VORBs) [Bull. Volcanol. 43 (1980) 675] observed in the floor zone, of the lake are fossil chimneys. In the roof zone, buoyant residual liquid both produced at the roof solidification front and gained from the floor as a result of incomplete convective mixing is envisaged to have percolated or ‘leaked‘ into the overlying highly-permeable cumulate, displacing less buoyant interstitial melt downward. The results from Rayleigh fractionation-type models formulated using boundary conditions based on a quantitative understanding of the convection in the magma indicate that most of the incompatible element variation over the height of the lake can be explained as a consequence of a combination of crystal settling and the extensive but imperfect convective mixing of buoyant residual liquid released from the floor solidification front. The remaining chemical variation is understood in terms of the additional influences of mush convection in the roof and floor on the vertical distribution of incompatible elements. Although cooling was concentrated at the roof of the lake, the floor zone is found to be thicker than the roof zone, implying that it grew more quickly. The large growth rate of the floor is explained as a consequence of a combination of the substantial sedimentation of olivine crystals and more rapid in situ crystallization due to both a higher liquidus temperature and enhanced cooling resulting from imperfect thermal and chemical mixing. 相似文献
13.
《Journal of Volcanology and Geothermal Research》2003,119(1-4):145-159
To understand how large submarine lava terraces form and why they are not commonly observed on land, we developed an isoviscous gravity flow model on an inclined surface to simulate the evolution and emplacement of lava flows under submarine conditions. By solving this preliminary model using a finite difference method, we are able to quantify how lava viscosity, pre-existing topographic slope, effusion rate, and lava volume affect meso-scale lava morphology. Our simulations show that, in general, high lava viscosity, gentle regional slope, and low effusion rate favor the formation of large terraces, but environmental conditions also play an important role. A gravity flow spreads more slowly underwater than subaerially. We also conclude that for low viscosity basaltic lava, the cooling of the lava body is one of the most critical factors that affect its shape. This study shows that the isoviscous model, though oversimplified, provides a quantitative tool to relate lava morphology to eruption characteristics. To gain a better understanding of the controls on submarine lava terrace formation, future models must take into account the temporal and spatial variation of lava viscosity, especially the effects of a brittle outer shell. 相似文献
14.
Factors which control lava flow length are still not fully understood. The assumption that flow length as mainly influenced by viscosity was contested by Walker (1973) who proposed that the length of a lava flow was dependent on the mean effusion rate, and by Malin (1980) who concluded that flow length was dependent on erupted volume. Our reanalysis of Malin's data shows that, if short duration and tube-fed flows are eliminated, Malin's Hawaiian flow data are consistent with Walker's assertion. However, the length of a flow can vary, for a given effusion rate, by a factor of 7, and by up to 10 for a given volume. Factors other than effusion rate and volume are therefore clearly important in controlling the lengths of lava flows. We establish the relative importance of the other factors by performing a multivariate analysis of data for recent Hawaiian lava flows. In addition to generating empirical equations relating flow length to other variables, we have developed a non-isothermal Bingham flow model. This computes the channel and levee width of a flow and hence permits the advance rates of flows and their maximum cooling-limited lengths for different gradients and effusion rates to be calculated. Changing rheological properties are taken into account using the ratio of yield strength to viscosity; available field measurements show that this varies systematically from the vent to the front of a lava flow. The model gives reasonable agreement with data from the 1983–1986 Pu'u Oo eruptions and the 1984 eruption of Mauna Loa. The method has also been applied to andesitic and rhyolitic lava flows. It predicts that, while the more silicic lava flows advance at generally slower rates than basaltic flows, their maximum flow lengths, for a given effusion rate, will be greater than for basaltic lava flows. 相似文献
15.
H. Wachendorf 《Bulletin of Volcanology》1973,37(4):515-529
Along the Lebombo monocline acid and basic magmas extruded alternatively as fissure eruptions, to a thickness of approximately 12 km during the time interval from the Triassic/Jurassic boundary to the Cretaceous. Textural evidence suggests that the rhyolites were emitted as lava flows. The rate of cooling or the grade of crystallization, respectively, produced a series of textural zones. The upper parts of the lava flow-units are intricately flow folded. It is postulated that the Lebombo rhyolites were generated in the upper mantle. 相似文献
16.
Martial Caroff Ccile Ambrics RenC. Maury Joseph Cotten 《Journal of Volcanology and Geothermal Research》1997,79(1-2)
The Bouzentès lava flow is a 20-m-thick alkali basalt flow emplaced during the last stage of formation of the Cantal stratovolcano at 4.2 Ma. Its upper part has 1- to 20-cm-thick vesicle-rich segregation sheets which recur every 0.1–2 m. These horizontal veins are hawaiitic in composition. They are characterized by hypertrophic development of their minerals (‘pegmatoids’) and by glassy phonolitic segregation vesicles. Internal differentiation within the Bouzentès lava flow was triggered by an unusually high water content, as suggested by pre-emptive iddingsite alteration of olivine phenocrysts. The proposed model of formation of the segregation sheets includes the upward motion of diapirs of residual melt plus addition of vapor from the bottom of the central liquid lens to the base of the upper solidified crust of the cooling lava flow. Olivine settling appears to have been inhibited or at least retarded by upward migration of melt plus vesicles. Most of the features observed in Bouzentès recall the internal differentiation processes usually described within thick Hawaiian lava lakes. The segregation vesicles are believed to result from an increase of gas solubility in residual melt during the crystallization process. 相似文献
17.
Andesitic–dacitic volcanoes exhibit a large variety of eruption styles, including explosive eruptions, endogenous and exogenous dome growth, and kilometer-long lava flows. The rheology of these lavas can be investigated through field observations of flow and dome morphology, but this approach integrates the properties of lava over a wide range of temperatures. Another approach is through laboratory experiments; however, previous studies have used higher shear stresses and strain rates than are appropriate to lava flows. We measured the apparent viscosity of several lavas from Santiaguito and Bezymianny volcanoes by uniaxial compression, between 1,109 and 1,315?K, at low shear stress (0.085 to 0.42?MPa), low strain rate (between 1.1?×?10?8 and 1.9?×?10?5?s?1), and up to 43.7 % total deformation. The results show a strong variability of the apparent viscosity between different samples, which can be ascribed to differences in initial porosity and crystallinity. Deformation occurs primarily by compaction, with some cracking and/or vesicle coalescence. Our experiments yield apparent viscosities more than 1 order of magnitude lower than predicted by models based on experiments at higher strain rates. At lava flow conditions, no evidence of a yield strength is observed, and the apparent viscosity is best approached by a strain rate- and temperature-dependent power law equation. The best fit for Santiaguito lava, for temperatures between 1,164 and 1,226?K and strain rates lower than 1.8?×?10?4?s?1, is $ \log {\eta_{\text{app}}} = - 0.738 + 9.24 \times {10^3}{/}T(K) - 0.654 \cdot \log \dot{\varepsilon } $ where η app is apparent viscosity and $ \dot{\varepsilon } $ is strain rate. This equation also reproduced 45 data for a sample from Bezymianny with a root mean square deviation of 0.19 log unit Pa?s. Applying the rheological model to lava flow conditions at Santiaguito yields calculated apparent viscosities that are in reasonable agreement with field observations and suggests that internal shear heating may be significant ongoing heat source within these flows, enabling highly viscous lava to travel long distances. 相似文献
18.
Active lava tubes have much higher temperatures than the surrounding rocks. Any change in the tube temperature produces a
change in the temperature distribution in the rocks and induces a thermoelastic deformation in them. We calculate such a deformation
by solving the equilibrium equation of linear thermoelasticity. We assume that the initial temperature distribution in the
medium is the steady-state solution of the heat equation for a very long cylindrical tube at constant temperature, embedded
in a medium with uniform thermal conductivity. We calculate the displacement and stress fields in the medium following to
a temperature change of the tube. A temperature increase produces a dilatation of the medium and a contraction of the tube,
while a temperature decrease produces the opposite effect. For a temperature change equal to 100 K, thermal stresses in the
order of 10 MPa are produced, which are large enough to fracture the rocks surrounding the tube. 相似文献
19.
R. W. Birnie 《Bulletin of Volcanology》1973,37(1):1-36
A Barnes PRT-5 radiation thermometer was used to obtain apparent surface temperatures of two Guatemalan volcanoes from land-based stations from 500 to 4000 meters distant. Isotherms of apparent surface temperatures, drawn on photographs of the volcanic terrain under study, delineate areas of fumarolic activity and active domal upgrowth. The excess radiant heat emitted from Pacaya Volcano is calculated from apparent surface temperatures corrected for atmospheric absorption of infrared radiation and for the adiabatic cooling of the atmosphere with altitude. The excess radiant heat data indicate that the lava flow extruded in June 1969 had completely solidified by December 1969. This calculation is consistent with theoretical estimates of the cooling of an extrusive lava sheet by conduction. Similar calculation of excess radiant heat emission shows the depth of the magma chamber underlying the Santiaguito Volcanic Dome to be 11 meters. This depth is consistent with field observations. Corrections are made for surface emissivity on Pacaya Volcano and the isotherms of real surface temperature plotted. Consideration is given to the times required for the equilibration of a geothermal gradient following the upward movement of a magma. 相似文献
20.
Ten carefully surveyed topographic profiles across a 1983 Royal Gardens basalt flow from the East Rift of the Kilauea Volcano were matched to digitally derived preflow profiles to construct accurate flow cross sections. Geometric parameters measured on these sections were then used to compute yield strengths and viscosities by means of several rheologic models. Calculated yield strengths (1.5–50 × 103 Pa) and viscosities (0.2–8.2 × 106 Pas) are comparable to earlier field estimates and slightly higher than laboratory determined values for aa basalt. Both yield strength and viscosity increased systematically downstream. The maximum observed temperature drop of 30 °C is insufficient to account for the 30-fold increase in yield strength, but could explain the three-fold order-of-magnitude increase in viscosity. The yield-strength increase downstream is more likely due to increasing crystallization and brecciation with time. For any cross section, calculations of rheologic parameters based on flow-margin depths generally gave lower values than those based on the dimensions of levees. This relationship may be attributed to the earlier formation and less complex evolution of the margins. The various equations gave more consistent results for upstream profiles, suggesting that calculations for remotely observed flows should avoid measurements near flow termini. 相似文献